An emulsion includes a continuous liquid phase and a liquid-phase material that is immiscible with the continuous phase and dispersed therein as particles. Examples of well-known emulsions include O-W type emulsions in which oil droplets are dispersed in a water-based continuous phase and W-O type emulsions in which, in contrast to the O-W type emulsions, water-based liquid droplets are dispersed in an oil-based continuous phase. Examples of known methods of producing such emulsions include surface-chemical methods that use an emulsifying agent and mechanical methods that use a special emulsification apparatus. Generally, stable emulsions are produced using a combination of these two types of methods. It is generally known that, when the latter mechanical methods are used, the use of different emulsification apparatuses causes significant differences in the properties of obtained emulsions (the size of the liquid droplets of the dispersed phase and the particle diameter distribution of the liquid droplets).
At present, emulsions are important raw materials and products in industrial areas relating to various products such as cosmetics, foods, paints, paper products, films, and recording materials. The particle diameter and particle diameter distribution of liquid droplets serving as the dispersed phase of an emulsion are important factors that influence on the stability of the emulsion and the properties of final products. More specifically, in emulsions for cosmetics, the differences in the average particle diameters and particle diameter distributions of the emulsified and dispersed liquid droplets result in a difference in spreadability on the skin. Moreover, the above differences greatly influence on the stability of the products.
Microcapsules are obtained by forming a polymer film or the like on the interface between the continuous and dispersed phases of an emulsion, and polymer fine particles are obtained by polymerizing the polymerizable dispersed phase of an emulsion. Such microcapsule or polymer fine particles are produced by subjecting an emulsion to treatment using a method including the steps of polymerization, filtration washing, drying, sieving, pulverization, and the like. These microcapsules and polymer fine particles are also used in various industrial areas. Microcapsules are used as information recording materials, such as toners for copying machines and printers, which utilize the pressure sensitivity, thermo-sensitivity, and photo-sensitivity of the microcapsules. Microcapsules are also used as display materials, such as electronic paper, and as pharmaceuticals, agricultural chemicals, insecticides, aromatics, thermal storage materials, and the like. Polymer fine particles are used as: anti-blocking agents for plastic films; optical materials used to impart light-diffusion and reflection prevention functions and used for spacer applications; paints and inks used to mat or color construction materials or the interior parts of automobiles, to improve the texture thereof, or to impart other functions; materials used to impart smoothness to cosmetic foundations and the like; additives used to improve thermal resistance and solvent resistance of resins or to impart various properties, such as low shrink property, thereto; and diagnostic reagents and fine particle preparations used in the medical field. Microcapsules and polymer fine particles are also used for other applications such as pigments, dyes, conductive components, thermal recording paper, reinforcing materials for resins, additives in fats and oils, artificial stones, and chromatography. In both microcapsules and polymer fine particles, the particle diameter and particle diameter distribution of the produced particles are determined substantially at the emulsification stage. Therefore, it is no exaggeration to say that the properties of the produced emulsion determine the performance of final products. Accordingly, there is a need to develop an emulsification apparatus that can easily manufacture a product having a desired average particle diameter and particle diameter distribution, in particular, a narrow particle diameter distribution, irrespective of in which form the product is used, as an emulsion, microcapsules, or polymer fine particles.
Various mechanical methods of manufacturing emulsions have been proposed. In the most general method, raw materials are charged into a batch-type tank and emulsified by stirring the materials in the tank using stirring blades rotating at fast speed.
In this method, a dispersion medium and a dispersive liquid are mixed at an appropriate ratio to prepare a preliminary emulsion. Then the preliminary emulsion is further reduced in particle diameter and emulsified using emulsification means such as a high-speed stirrer (dissolver), homogenizer, or in-line mixer to thereby produce a stable emulsion. With such methods, high energy can be applied to an oil-based liquid when it is dispersed in a water-based medium. These methods are particularly effective to obtain an emulsion having a particle diameter of less than 10 μm.
However, in the above methods, during the dispersion of the oil-based liquid, the coalescence of the droplets thereof occurs frequently, and the dispersion and coalescence are repeated. Therefore, when the oil-based liquid contains fine particles therein, these fine particles may be transferred into the water-based medium, and this may cause a reduction in the amount of the fine particles in the oil-based liquid. Moreover, the water-based medium may be contaminated by the transferred fine particles. When the droplets of the oil-based liquid are formed into microcapsules, the transferred fine particles may adhere to the shell surfaces of the microcapsules during the formation thereof, so that the microcapsules themselves may be contaminated. In addition, since the shear force necessary for emulsification is exerted only on areas in close proximity to the stirring blades, the shear force is non-uniform, depending on the distance from the stirring blades. This may cause the broadening of the particle diameter distribution of the dispersed liquid droplets. Moreover, it is unfortunately difficult to scale up.
In some apparatuses, for example, a stirring unit for allowing the solutions to flow over the entire tank is attached separately from the stirring blades to prevent the above problems. However, it is very difficult to completely eliminate the problems. Moreover, when scale-up is performed, the stirring blades and the driving unit therefor increase in size and become expensive. The rotating members driven at high speed have precise structures, and this is disadvantageous for maintenance. When a large amount of materials must be emulsified, the emulsification operation takes a long time. In such a case, the materials can be denatured during the emulsification operation.
To solve the problems in the batch-type emulsification methods, continuous emulsification methods have been proposed.
For example, Japanese Patent Application Laid-Open No. Hei 5-49912 discloses an emulsification method in which raw materials for emulsification are emulsified by rotating a cylindrical rotor having protruding edges on its outer wall inside a cylindrical stator having protruding edges on its inner wall to apply a shear force to the raw materials while the raw materials are allowed to pass through the gap between the stator and the rotor. In this method, the strength of the shear force is determined by the rotation speed of the rotor. Therefore, when a large shear force is required, i.e., an emulsion containing small liquid droplets of the dispersed phase is produced, a very large power unit is required. Moreover, when the amount of the emulsion produced per unit time is increased, the residence time of the raw materials for emulsification in the emulsification apparatus decreases. This causes the problem in that an emulsion including a dispersed phase having a uniform particle diameter distribution is not obtained. In addition, since the shapes of the protrusions are complicated and the gap between the inner wall of the stator and the outer wall of the rotor is very small, the machining and maintenance of the apparatus are difficult.
Japanese Patent Application Laid-Open No. Hei 6-142492 (U.S. Pat. No. 5,554,323) discloses a method of manufacturing microcapsules. In this method, preliminary emulsification is performed under stirring. Then a double cylinder-type continuous emulsification apparatus is used to produce an emulsion having a broad particle diameter distribution by changing the rotation speed of the inner cylinder of the apparatus continuously or stepwise. There is a description that an emulsion having a broad particle diameter distribution can be manufactured using this method. The feature of the method is that the emulsion does not contain excessively large particles and excessively small particles. However, in this method, the amounts of raw materials charged and the rotation speed of the inner cylinder of the emulsification apparatus must be controlled, and therefore the operation is complicated. In addition, when the materials to be emulsified are reactive, the apparatus can be clogged.
Japanese Patent Application Laid-Open No. Hei 9-029091 (U.S. Pat. No. 5,785,423) discloses a continuous emulsification method comprising feeding an oil phase solution continuously from the bottom of an emulsification tank including stirring blades, feeding a water phase solution continuously from the lower side-portion of the emulsification tank, and discharging an emulsion continuously from the upper portion of the emulsion tank. There is the description that this method can prevent the clogging of the emulsification apparatus even when the raw materials for emulsification are reactive compounds. However, when the rate of emulsification is increased, the particle diameter distribution of the dispersed phase deteriorates also in this method. In the worst case, non-emulsified raw materials may be discharged through a short path.
Japanese Patent Application Laid-Open No. Hei 5-212270 discloses a continuous emulsification method using a porous glass pipe. With this method, the apparatus used is expensive. When the raw materials are reactive, the porous glass pipe may be clogged. Moreover, the particle diameter of the emulsion is determined by the pressure used to extrude the raw material to be emulsified from the porous glass pipe and the flow state of a fluid that forms the continuous phase. Therefore, the operation conditions for controlling the particle diameter are complicated. Another problem is that, since the porous glass pipe is expensive, extra cost is required for scale-up.
Japanese Patent Application Laid-Open Nos. Hei 2-261525 and Hei 9-201521 disclose methods and apparatuses for instantaneously emulsifying raw materials for emulsification by collision at ultrahigh pressure and high speed. In these apparatuses, the operating pressures thereof are very high, so the apparatus bodies must have rigid structures. Another problem is that the degree of abrasion of the apparatus is high. The emulsification in the above apparatus is based on the impact force of the collision of the raw materials for emulsification and is difficult to control. One disadvantage of these methods and apparatuses is that the particle diameter distribution of the liquid droplets of the dispersed phase in the emulsion is highly non-uniform.
Japanese Patent Application Laid-Open No. 2000-254469 (U.S. Pat. No. 6,379,035) discloses a static mixing and stirring apparatus having a structure in which a plurality of disk-shaped elements having a plurality of hole portions (having, for example, a truncated polygonal pyramid shape or truncated conical shape) are disposed in a cylindrical case at predetermined intervals in their thickness direction. Japanese Patent Application Laid-Open No. 2002-28463 discloses a fluid mixer that includes a cylindrical body having a rectangular cross-section and a plurality of pairs of first and second assembled plate bodies that are fitted inside the cylindrical body. The first assembled plate body includes a quadrilateral base plate and solid hollow pentagonal bodies continuously arranged thereon and has a through hole at the center, and the second assembled plate body has a recessed portion of any shape at the center. These apparatuses are mixers for liquid but can be used as emulsification apparatuses. However, some problems of these apparatuses include not only that the shapes of the elements used are complicated but also that the arrangement of the elements must be precisely adjusted in the apparatus.
Japanese Patent Application Laid-Open No. 2002-159832 (US2002/060950A1) discloses an emulsion producing apparatus comprising mixing means for mixing a plurality of liquids with each other, a pressurizing pump for pressurizing the mixture liquid, and emulsifying means for bringing the mixture liquid pressurized by and delivered from the pressurizing pump into an emulsified state. The emulsifying means includes a plurality of chambers partitioned by partition walls each having at least one small hole, and the mixture liquid flows into the plurality of chambers. In this apparatus, raw materials for emulsification are injected from the small hole into an adjacent space at high speed and high pressure. The raw materials for emulsification are pulverized and destroyed by a strong impact force at the time of injection and are thereby emulsified. More specifically, only the destroying phenomenon by the impact is used as the emulsification principle. Since the destroying phenomenon by the impact is difficult to control, the particle diameter of the obtained emulsion tends to be non-uniform. Moreover, since a high pressure is used for injection, the emulsification apparatus must have a rigid structure.
As has been described, the continuous emulsification methods and apparatuses proposed previously are not satisfactory because of poor uniformity of the liquid droplets of the dispersed phase in obtained emulsions, difficulty in scale-up, complexity of the apparatus and their maintenance, and other reasons.